Pressure swing adsorption is a well-known method for the separation of bulk gas mixtures and for the purification of gas streams containing undesirable impurities. The method has been developed and adapted for a wide range of feed gases, operating conditions, product recovery, and product purity. Most large pressure swing adsorption (PSA) systems utilize multiple parallel adsorber beds operated in staggered sequential cycles using typical process steps of feed/adsorption, pressure equalization, depressurization, provide purge, purge, and repressurization. These PSA systems are widely used in the chemical process industries for the recovery and purification of valuable gaseous products such as hydrogen, carbon oxides, synthesis gas, light hydrocarbons, and atmospheric gases.
The design of these PSA systems can present complex operating challenges because of the large number of switching valves required for the cyclic operation of multiple adsorber beds. For example, a relatively simple four-bed PSA system has at least 24 valves while a large ten-bed PSA system may have up to 67 valves. The operation of PSA systems to meet design specifications for product recovery and purity requires that these valves operate reliably of over long periods of time. All valves eventually will experience some type of malfunction, however, and replacement will be necessary. Valve testing and replacement may be carried out during scheduled maintenance outages, but it is likely that some valve malfunctions will occur between scheduled outages.
Valve malfunctions may include, for example, failure of a valve to open or close completely, slow valve response to control signals, no valve response to control signals, and leakage to the atmosphere due to faulty stem seals. These types of malfunctions can be detected relatively easily by skilled plant operators. Another type of valve malfunction is the leakage of gas across a closed valve due to seal degradation. This type of malfunction is much more difficult to detect than the other type of malfunctions because it generally occurs slowly and therefore is not immediately obvious to the plant operators. When the operators finally realize that a valve is leaking, which could take weeks or even months, it is a difficult and time-consuming task to determine which valve is leaking from analysis of available process information. This type of leak can cause a drop in performance of the PSA plant due to unbalanced operation of the adsorbers or due to a direct loss of product gas. In addition, such a leak eventually can progress to a point at which the PSA plant trips off-line, for example, because the concentration of impurities in the product gas exceeds specified limits. This results in unplanned downtime and an economic penalty for the plant owner.
In order to identify the valves that have a sealing defect in the closed position, seal tests are conventionally employed during periodic maintenance shutdowns. However, this type of inspection is time-consuming, laborious, and extends downtime, thereby resulting in lost production. In addition, the time between maintenance shutdowns may be several months to two or more years.
There is a need in the field of PSA gas separation technology for improved operating methods to detect and identify leaking valves while the plant is in operation between regular maintenance shutdowns. This need is addressed by the embodiments of the present invention described below and defined by the claims that follow.